Semiconductor device and method for fabricating same

ABSTRACT

A method is provided with:
         arranging nitrogen atoms on a surface of a silicon substrate;   performing a heat treatment in a hydrogen atmosphere so that the nitrogen atoms and silicon atoms existing on the surface of the silicon substrate are brought into a three-coordinate bond state; and   forming a silicon oxide film on the silicon substrate with the three-coordinate bond state of nitrogen atoms and the silicon atoms being maintained.

This application is a continuation-in-part of U.S. patent applicationSer. No. 10/127,571.

BACKGROUND OF THE INVENTION

1. Technical Field of the Invention

The present invention relates generally to a semiconductor device havingan insulating film, and a method for fabricating the same.

2. Description of Related Art

Since a typical semiconductor oxide film has a very great part as aninsulating film in various semiconductor devices, the qualities andforming methods thereof have been variously studied. As a method forforming a semiconductor oxide film, a so-called thermal oxidationprocess for exposing the surface of a semiconductor to oxygen moleculargas in the atmosphere at high temperatures has been widely used. Withthe scale down of elements, it is considered that the thickness of thethermal oxide film is decreasing. However, if the thickness of the oxidefilm is 2 nm or less, the current tunneling through the oxide filmrapidly increases to cause a phenomenon that impurities pass through theoxide film to diffuse. For that reason, it is being difficult to improvethe performance of elements due to the scale down.

Therefore, there is considered a method for mixing nitrogen in an oxidefilm to form an oxynitride film. If nitrogen atoms are introduced intoan oxide film, the dielectric constant of the oxynitride film increases,so that the thickness of an oxynitride film having the same capacitanceas that of an oxide film can be larger. In addition, since the diffusionof impurities, such as boron, can be suppressed, it has been possible toeffectively form a thinner high-performance insulating film by theconversion to an oxide film.

However, if nitrogen atoms are introduced into an oxide film, energylevels due to nitrogen atoms are formed in a band gap in the insulatingfilm although the effective thickness can be decreased. For that reason,if an oxide film into which nitrogen atoms are introduced is used as,e.g., a gate insulating film for a MOS transistor, current drivabilitydecreases due to a degradation of carrier mobility. In order to preventthis, there is considered a method for preventing the scattering ofelectrons by localizing introduced nitrogen atoms in the vicinity of thesurface of the gate insulating film so as to be spaced from theinterface between the semiconductor layer and the gate insulating film.However, it is difficult for this method to completely control thedoping amount and to reduce the energy level due to nitrogen atoms.

Japanese Patent Laid-Open Publication No. 2001-203198 discloses a methodfor forming an oxynitride film. In this method, the surface of a siliconsubstrate is hydrogen-terminated, and hydrogen atoms are removed by heattreatment. Thereafter, nitrogen atoms and oxygen atoms are absorbed ontounbonded bonds in a heating atmosphere of NO gas or NO+O₂ to form amonoatomic oxynitride layer. Thereafter, it is oxidized in theatmosphere to form an oxynitride film having an oxide layer on the sideof the silicon substrate and an oxynitride layer on the side of thesurface. However, most of nitrogen atoms in the oxynitride film formedby this method are in a two-coordinate bond state, so that it isdifficult to reduce the energy level due to nitrogen atoms in the bandgap of the oxynitride film.

SUMMARY OF THE INVENTION

A method for fabricating a semiconductor device according to a firstaspect of the present invention includes: arranging nitrogen atoms on asurface of a silicon substrate; performing a heat treatment in ahydrogen atmosphere so that the nitrogen atoms and silicon atomsexisting on the surface of the silicon substrate are brought into athree-coordinate bond state; and forming a silicon oxide film on thesilicon substrate with the three-coordinate bond state of nitrogen atomsand the silicon atoms being maintained.

A method for fabricating a semiconductor device according to a secondaspect of the present invention includes: performing a heat treatment ona silicon substrate in a first hydrogen atmosphere; arranging nitrogenatoms on a surface of the silicon substrate; performing a heat treatmentin a second hydrogen atmosphere so that the nitrogen atoms and siliconatoms existing on the surface of the silicon substrate are brought intoa three-coordinate bond state; and forming a silicon oxide film on thesilicon substrate with the three-coordinate bond state of the nitrogenatoms and the silicon atoms being maintained.

A semiconductor device according to a third aspect of the presentinvention includes a silicon oxynitride film having a thickness of 2 nmor less, the silicon oxynitride film including an oxynitride layer whichis formed on at least a surface of a silicon substrate and in whichnitrogen atoms are in a three-coordinate bond state, and a silicon oxidelayer which is formed between the oxynitride layer and the siliconsubstrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart showing fabricating steps of a method forfabricating a semiconductor device according to the first embodiment ofthe present invention;

FIG. 2 is a block diagram showing the construction of an oxynitride filmforming apparatus for use in the fabricating steps in the firstembodiment;

FIG. 3 is a sectional view showing the state where an oxynitride layeris formed in the first layer on the surface in the middle of thefabricating steps in the first embodiment;

FIG. 4 is a sectional view showing the state where an oxynitride layeris formed in the first layer on the surface and an oxide layer is formedin the second layer from the surface in the middle of the fabricatingsteps in the first embodiment;

FIG. 5 is a sectional view showing the construction of a MOSFET formedby a fabricating method according to the second embodiment of thepresent invention;

FIG. 6 is a graph showing the relationship between fixed charges and themobility of electrons on the interface between a silicon substrate and asilicon oxynitride film;

FIG. 7 is a sectional view showing the state where nitrogen atoms are ina three-coordinate bond state on the surface of a silicon substrate inthe middle of the fabricating steps in the third embodiment;

FIG. 8 is a sectional view of a semiconductor device fabricated by afabricating method according to the third embodiment of the presentinvention;

FIG. 9 is a sectional view showing the fabricating steps of afabricating method according to the fourth embodiment of the presentinvention;

FIG. 10 is a sectional view of a semiconductor device after a firstnitriding and annealing step is carried out;

FIG. 11 is a sectional view of a semiconductor device after a secondnitriding and annealing step is carried out;

FIG. 12 is a flow chart showing the fabricating steps of a fabricatingmethod according to the fifth embodiment of the present invention;

FIG. 13 is a timing chart of a heat treatment in the fabricating methodaccording to the fifth embodiment of the present invention;

FIG. 14 is a flow chart showing the fabricating steps of a fabricatingmethod according to the sixth embodiment of the present invention;

FIG. 15 is a timing chart of a heat treatment in the fabricating methodaccording to the sixth embodiment of the present invention; and

FIG. 16 is a timing chart of a heat treatment in a modification of thefirst to fourth embodiments.

DESCRIPTION OF THE EMBODIMENTS

Referring now to the accompanying drawings, the embodiments of thepresent invention will be described below.

First Embodiment

Referring to FIGS. 1 through 4, a method for fabricating a semiconductordevice according to the first embodiment of the present invention willbe described below. The semiconductor device fabricating method in thisfirst embodiment is a method for fabricating a semiconductor devicehaving an oxynitride film, and the steps of fabricating an oxynitridefilm are shown in FIG. 1. The fabrication of the oxynitride film in thisfirst embodiment is carried out by using an oxynitride film formingapparatus shown in FIG. 2. Before describing the fabricating method inthe first embodiment, the oxynitride film forming apparatus shown inFIG. 2 will be described below.

As shown in FIG. 2, this oxynitride film forming apparatus has a chamber4 having a heating furnace 3 for housing therein a movable susceptor 2for supporting thereon a plurality of semiconductor substrates 1. Tothis chamber 4, an NO gas source 5, a nitrogen gas source 6 and anoxygen gas source 7 are connected as atmospheric gas sources. Thechamber 4 is provided with a gas inlet 8 for feeding NO gas, nitrogengas and oxygen gas into the chamber 4 from these gas sources, and a gasoutlet 9 for discharging gases. On the NO gas source 5, nitrogen gassource 6 and oxygen gas source 7, valves 10, 11 and 12 are mounted,respectively, for enabling the control of the partial pressures ofgases. A heater 13 is provided around the chamber 4 to be controlled bya temperature control unit (not shown).

Referring to FIGS. 1 and 2, the first embodiment of a fabricating methodaccording to the present invention will be described below.

First, as shown at step Si in FIG. 1, a silicon substrate 1 having (100)plane as a principal plane is treated with diluted hydrofluoric acid, sothat the surface of the silicon substrate terminates with hydrogen.Then, as shown at step S2 in FIG. 1, the silicon substrate terminatingwith hydrogen is mounted on the susceptor 2, and the valves 10, 11 and12 are open and closed at room temperatures so that the atmosphere inthe chamber 4 is only nitrogen gas. Subsequently, the heater 13 providedaround the chamber 4 is controlled to raise the temperature of thesilicon substrate 1 to 600° C. to completely remove hydrogen from thesilicon substrate 1 (see step S2 in FIG. 1).

Thereafter, as shown at step 53, the heater 13 is controlled to lowerthe temperature of the silicon substrate 1 to 200° C. Moreover, thevalve 10 is open and closed to mix NO gas with a partial pressure of10⁻⁶ Torr (=10⁻⁶×133.322 Pa) to hold it for one minute. Thus, an oxidefilm containing nitrogen atoms (an oxynitride film) is formed on thefirst layer of the silicon substrate 1.

Then, as shown at step S4, the valves 10, 11 and 12 are open and closedagain so that the atmosphere in the chamber 4 is only nitrogen gas, andthe heater 13 is controlled to raise the temperature of the siliconsubstrate 1 to 900° C. to hold it for one minute. Thus, as shown in FIG.3, nitrogen atoms in a first layer 15 of the silicon substrate 1 aresubstantially in a three-coordinate bond state to form a stablestructure. Furthermore, the temperature range for providing athree-coordinate bond state is preferably in the range of from 600° C.to 950° C.

Subsequently, the valves 10, 11 and 12 are open and closed so that thepartial pressure of oxygen in the atmosphere in the chamber 4 is raisedto the same level as that in the atmosphere, and the temperature of thesilicon substrate 1 is raised to 800° C. to be held for ten minutes.Thus, as shown in FIG. 4, an oxynitride layer is formed in the firstlayer 15 on the surface of the silicon substrate 1, and an oxide layer16 is formed in the second and subsequent layers below the first layer15. That is, nitrogen atoms in the first layer 15 on the surface of thesilicon substrate 1 are maintained in a three-coordinate bond state toform an oxynitride film on the surface of the silicon substrate and toform the oxide layer 16 in the second and subsequent layers. Thus, asilicon oxynitride film is formed, which has the first layer 15 to be anoxynitride layer and the second layer 16 and subsequent layers to beoxide layers, and which has an interface structure being a flatstructure at an atomic level and a thickness of 20 Å (=2 nm) or less.Thus, the energy levels in the band gap are greatly reduced.Furthermore, although the oxynitride film is formed on the surface ofthe silicon substrate 1, it is preferably formed on the uppermostsurface, i.e., the first layer.

As can be understood from FIG. 4, in a semiconductor device fabricatedby the fabricating method of this embodiment, the atoms that are theclosest to a nitrogen atom in a three-coordinate bond state are siliconatoms, and the atoms that are next closest are atoms other than siliconatoms, e.g., oxygen atoms.

As described above, according to the fabricating method in thisembodiment, the oxynitride film having the three-coordinate-bondednitrogen is formed, and the second and subsequent layers from thesurface are complete oxide layers. Therefore, it is possible to obtainan oxynitride film wherein the interface structure is a flat structureat an atomic level, and it is possible to obtain a silicon oxynitridefilm (insulating film) wherein the energy level due to nitrogen atoms inthe band gap is reduced.

Generally, when the gate insulating film is formed of only a siliconoxide film and has a thickness of 2 nm or less, the impurities dopedinto the gate electrode (in particular boron (B)) diffuse into thesilicon substrate. However, when the silicon oxynitride film of thesemiconductor device fabricated in accordance with the fabricatingmethod of this embodiment is used as a gate insulating film of a MOSFET,even if the thickness of the gate insulating film is 2 nm or less, it ispossible to prevent the impurities doped into the gate electrode (inparticular boron (B)) from diffusing into the silicon substrate sincethe degree of diffusion of boron in a silicon oxynitride film is lessthan that in a silicon oxide film.

Furthermore, nitrogen molecular radicals may be substituted for nitrogengas.

While it has been exposed to an atmosphere of oxygen at 800° C. for tenminutes in this embodiment, various temperatures and times can becombined as long as the oxynitride film having thethree-coordinate-bonded nitrogen in the first layer of the siliconsubstrate 1 is formed, and materials other than oxygen can be used aslong as an oxidization reaction occurs.

While hydrogen on the surface of the silicon substrate 1 is removed atthe heating step before the oxidizing step, the removal of hydrogen maybe carried out at a step other than the heating step.

In the place of nitrogen gas, a gas of a kind which does not react withthe semiconductor, e.g., an inert gas such as argon gas, may be used. Asemiconductor causing an oxidation reaction on a portion other than thesilicon substrate may be used as a semiconductor used in thisembodiment.

Second Embodiment

Referring to FIG. 5, a method for fabricating a semiconductor deviceaccording to the second embodiment of the present invention will bedescribed below. The fabricating method in this second embodiment usesthe fabricating method in the first embodiment to form a gate oxide filmof a MOSFET. First, as shown in FIG. 5, a field oxide film 18 is formedon a silicon substrate 1. On the surface of an element region isolatedby the field oxide film 18, a gate oxynitride film 19 is formed. In thiscase, the gate oxynitride film 19 having a thickness of 20 Å or less isformed by the oxynitride film forming method described in the firstembodiment. Thereafter, a gate electrode material is deposited thereonand patterned to form a gate electrode 20. Subsequently, the gateelectrode 20 is used as a mask to form a source region 21 a and a drainregion 21 b by ion implantation to obtain a MOSFET.

The gate oxynitride film 19 thus formed has a very uniform interface. Asa result, the obtained MOSFET has a small variation in thresholdvoltage, no deterioration of mobility, and stable characteristics.

Furthermore, in the oxynitride films formed by the above describedfabricating methods in the first and second embodiments, most ofnitrogen atoms in the oxynitride films have the three-coordinate bonds.The allowable contents of nitrogen atoms of coordinate bonds differentfrom the three-coordinate bonds will be described below. If all ofnitrogen atoms have the three-coordinate bonds, the fixed charge iszero, whereas if there are nitrogen atoms of coordinate bonds differentfrom the three-coordinate bonds, the fixed charge occurs. Therefore, theamount of nitrogen atoms of coordinate bonds different from thethree-coordinate bonds can be defined by the amount of fixed chargessince it is in proportion to the amount of fixed charges. If therelationship between the amount of fixed charges and the mobility ofelectrons on the interface between the silicon oxynitride film producedin the above described embodiment and the silicon substrate issimulated, the characteristic graph shown in FIG. 6 is obtained. As canbe seen from this characteristic graph, the mobility of electronsrapidly deteriorates if the amount of fixed charges exceeds 1.0×10¹¹(cm⁻²) to 2.0×10¹¹ (cm⁻²). Therefore, in order to prevent thedeterioration of the performance of the element, the amount of fixedcharges is preferably 2.0×10¹¹ (cm⁻²) or less. Furthermore, in the abovedescriptions, the amount of fixed charges is the value on the interfacebetween the silicon oxynitride film and the silicon substrate. However,it has been understood that the amount of fixed charges hardly varies ina range of 10 Å from the above described interface in a directionperpendicular to the silicon oxynitride film.

Third Embodiment

Referring to FIGS. 7 and 8, a method for fabricating a semiconductordevice according to the third embodiment of the present invention willbe described below.

In the above described first and second embodiments, the method forforming the oxynitride film using NO gas has been described. In thefollowing embodiment, a method for forming an oxide film after arrangingnitrogen atoms on the surface of a silicon substrate will be described.

In the fabricating method of this embodiment, nitrogen atoms arearranged on the surface of a silicon substrate prior to the abovedescribed usual oxidation. For example, nitrogen atoms are introduced tothe surface of the silicon substrate by plasma nitriding, andthereafter, a high-temperature heating process is carried out. FIG. 7schematically shows nitrogen atoms and silicon atoms on the surface ofthe silicon substrate 1 after the high-temperature heating process. Ascan be seen from FIG. 7, nitrogen atoms on the surface of the siliconsubstrate 1 are in the three-coordinate bond state with adjacent siliconatoms.

Subsequently, an oxidation process is carried out under a reducedpressure to oxidize the silicon substrate 1. Oxidizing conditions can beoptionally set. In this embodiment, an oxide film having a thickness of1 nm was formed at an oxygen partial pressure of 40 Torr at atemperature of 700° C. FIG. 8 schematically shows the bonding state ofoxygen atoms, nitrogen atoms and silicon atoms on the surface of thesilicon substrate 1 and in the oxide film after oxidation. As can beseen from FIG. 8, since nitrogen atoms are in the three-coordinate bondstate with silicon atoms, the configuration of nitrogen atoms isenergetically stable, and the change of state is not carried out even inthe oxidation reaction, i.e., the three-coordinate bond state is held,so that nitrogen atoms exist on the surface of the oxide film. Moreover,since nitrogen atoms form stable three-coordinate bonds with siliconatoms, a high quality of insulating film having a small amount of energylevels in the gap is formed similar to the first embodiment.

Fourth Embodiment

Referring to FIG. 9, a method for fabricating a semiconductor deviceaccording to the fourth embodiment of the present invention will bedescribed below. The fabricating process in this fourth embodiment is amethod for fabricating a MOSFET, and forms a gate insulating film usingthe fabricating method in the third embodiment.

First, as shown in FIG. 9( a), nitrogen atoms are arranged on part ofthe surface of the silicon substrate 1 to form a region 30, in whichnitrogen atoms are in the three-coordinate bond state with siliconatoms, by a high-temperature heat treatment. In this embodiment,nitrogen atoms are introduced into the silicon substrate 1 at a lowenergy (10 eV) with plasma so that the density of nitrogen is 5.0×10¹⁴cm⁻². In addition, in order for nitrogen to be in the three-coordinatebond state, annealing was carried out at 950° C. for thirty minutes.Furthermore, a method for introducing nitrogen atoms onto the surface ofthe silicon substrate 1 may use a nitrogen atom containing gas, or use agas having a reducing action to cause nitrogen atoms to remain on thesurface of the silicon substrate after an oxygen containing gas, such asNO gas, is used for introducing nitrogen. In this embodiment, the heattreatment was carried out at 950° C. for thirty minutes as a step ofcausing the three-coordinate bond state. However, temperature and timecan be optionally set.

Then, as shown in FIG. 9( b), the surface of the silicon substrate 1 isoxidized to form an insulating film (oxynitride film) 32 on the surfaceof the silicon substrate 1. In this embodiment, the oxide film having athickness of 1 nm was formed at 700° C. at an oxygen partial pressure of40 Torr. However, the oxidation atmosphere, time and the partialpressure of gas can be optionally set, and the radical oxidation or thelike can be used.

Then, as shown in FIG. 9( c), a polysilicon film 34 is deposited on theinsulating film 32. Subsequently, as shown in FIG. 9( d), thelithography technique or the like is used for forming a photoresistpattern 36 on the polysilicon film 34, and the photoresist pattern 36 isused as a mask for patterning the polysilicon film 34 to form a gateelectrode 34 a (see FIG. 9( d)).

Then, as shown in FIG. 9( e), the photoresist pattern 36 and the gateelectrode 34 a are used as a mask for implanting impurity ions to form asource region 38 a and a drain region 38 b.

Then, as shown in FIG. 9( f), after the photoresist pattern 36 isremoved, annealing is carried out for activation. Thereafter, theformation of an insulating film between wiring portions and theformation of wiring portions are carried out by a usual method tocomplete a semiconductor device.

As described above, also in this fourth embodiment similar to the thirdembodiment, nitrogen atoms form stable three-coordinate bonds withsilicon atoms, so that a high quality gate insulating film having asmall amount of energy levels in the gap is formed. Thus, it is possibleto form a uniform and stable MOSFET wherein the variation in thresholdvoltage is small and the mobility does not deteriorate.

Furthermore, in the above described third and fourth embodiments,nitrogen atoms in the insulating film are substantially in thethree-coordinate bond state. Similar to the first and secondembodiments, the allowable contents of nitrogen atoms of coordinatebonds different from the three-coordinate bonds in the insulating filmare defined by the amount of fixed charges, and the amount of fixedcharges is preferably 2.0×10¹¹ (cm⁻²) or less in order to prevent thedeterioration of the performance of the element.

In the above described first through fourth embodiments, the step ofnitriding the surface of the silicon substrate 1 to provide thethree-coordinate bond state by annealing is carried out only one time.However, after a step of nitriding the surface of the silicon substrate1 to provide the three-coordinate bond state by annealing as shown inFIG. 10 is carried out one time and before the silicon oxide film isformed, a step of nitriding the surface of the silicon substrate 1 tocarry out annealing as shown in FIG. 11 may be repeated at least onetime.

By repeating the step of nitriding the surface of the silicon substrate1 to provide the three-coordinate bond state by annealing, it ispossible to enhance the density of nitrogen atoms in thethree-coordinate bond state on the surface of the silicon substrate 1,so that it is possible to obtain an insulating film wherein energylevels in the band gap are reduced.

Fifth Embodiment

Referring to FIG. 12 and FIG. 13, a method for fabricating asemiconductor device according to the fifth embodiment of the presentinvention will be described below. The fabricating method in this fifthembodiment is a method for fabricating an oxynitride film. FIG. 12 showsthe fabricating method, and FIG. 13 shows a timing chart of a heattreatment.

First, as shown at step S11 of FIG. 12, the surface of the siliconsubstrate is nitrided, i.e., nitrogen atoms are arranged on the surfaceof the silicon substrate. For example, as explained in the descriptionsin the third embodiment, nitrogen atoms are introduced to the surface ofthe silicon substrate by plasma nitriding, and thereafter, a heattreatment (annealing) is performed at a high temperature. As shown inFIG. 13, the annealing is performed at a temperature in the range offrom 400° C. to 500° C. As explained in the descriptions in the thirdembodiment, the nitrogen atoms at the surface of the silicon substrateare in a three-coordinate bond state with the silicon atoms adjacentthereto. The plasma nitriding can be replaced by nitriding using NH₃gas.

Next, a heat treatment is performed in an H₂ atmosphere, as shown instep S12 of FIG. 12. An effect of performing a heat treatment in an H₂atmosphere is to facilitate the movement of silicon atoms and nitrogenatoms on the surface of the silicon substrate, thereby effectivelychanging the state of the nitrogen atoms to the three-coordinate bondstate, which is stable. Another effect is that since the oxygen atomsnear the surface are removed during the heat treatment in the H₂atmosphere, the nitrogen atoms are brought into the stable coordinationstate without being hindered by oxygen atoms. The heat treatmentconditions in the H₂ atmosphere can be optionally set. However, it ispreferable that the temperature is set to be equal to or less than atemperature at which nitrogen (N) atoms are removed (950° C.) and equalto or more than a temperature at which nitrogen and oxygen atoms canmove on the surface (500° C.), and that the partial pressure of H₂ gasis set to be equal to or more than a pressure at which nitrogen atomsare removed (50 Torr). Considering these conditions, the heat treatmentconditions of this embodiment are set as shown in FIG. 13, i.e., atemperature of 500° C. or more and 950° C. or less, and an H₂ partialpressure of 50 Torr or more. When the pressure is less than 50 Torr, thesurface becomes coarse since SiH is removed.

After the heat treatment in the H₂ atmosphere, the silicon substrate isoxidized (step S13 of FIG. 12). Although oxidation conditions can beoptionally set, in this embodiment, the oxygen partial pressure is setto be 40 Torr and the temperature is set to be 700° C., as shown in FIG.13. After the oxidation, an oxide film having a thickness of 1 nm isformed. The bonding state of oxygen, nitrogen, and silicon atoms on thesurface of the silicon substrate and in the oxide film after theoxidation is the same as that of the third embodiment shown in FIG. 8.As can be seen from FIG. 8, since nitrogen atoms are in thethree-coordinate bond state with silicon atoms, the state of nitrogenatoms is energetically stable, and the change of state is not carriedout even in the oxidation reaction, i.e., the three-coordinate bondstate is held, so that nitrogen atoms exist on the surface of the oxidefilm. Moreover, since nitrogen atoms are in the stable three-coordinatebonds with silicon atoms, a high-quality insulating film having a smallamount of energy levels in the gap is formed as in the case of the firstembodiment.

In the descriptions of FIG. 12, the surface nitriding, the heattreatment in the H₂ atmosphere, and the oxidation were separatelyexplained. Although these steps can be performed in separate devices, itis desirable that these steps be sequentially performed in order to forman insulating film having a better film quality. Such sequentialprocessing will be described as the sixth embodiment below.

Sixth Embodiment

Referring to FIG. 14 and FIG. 15, a method for fabricating asemiconductor device according to the sixth embodiment of the presentinvention will be described below. This embodiment includes step S10 forperforming a heat treatment in an H₂ atmosphere before step S11 fornitriding the surface of the silicon substrate in the fifth embodiment.

In step S10, the defects on the surface of the silicon substrate areremoved by the heat treatment in the H₂ atmosphere. Accordingly, thesurface of the substrate is flattened. The heat treatment conditions inthe H₂ atmosphere in step S10 can be optionally set. However, it ispreferable that the temperature be equal to or more than a temperature(500° C.) at which silicon atoms can move on the surface of the siliconsubstrate, and the partial pressure of H₂ gas be equal to or more than apressure (50 Torr) at which silicon atoms are removed. Considering theseconditions, the heat treatment conditions of step S10 in this embodimentare set as follows: a temperature of 500° C. or more; and an H₂ partialpressure of 50 Torr or more. Because of the heat threatment at step S10,oxygen is removed from the native oxide layer on the surface of thesilicon substrate.

After the heat treatment of step S10, step S11 (surface nitriding), stepS12 (heat treatment in the H₂ atmosphere) and step S13 (oxidation)explained in the descriptions of the fifth embodiment are performed. Inthis embodiment, the heat treatment in the H₂ atmosphere in step S12 isperformed under the conditions of a temperature of from 500° C. to 950°C., and a partial pressure of approximately 50 Torr. As in the case ofthe fifth embodiment, the heat treatment conditions in the H₂ atmospherein this embodiment are set as follows: a temperature (500° C. or more)at which nitrogen (N) and silicon (Si) atoms can move on the surface ofthe substrate and a temperature (950° C. or less) at which nitrogen (N)atoms do not cohere; and a pressure (50 Torr or more) at which nitrogen(N) atoms are not removed from the surface of the substrate. When thepressure is less than 50 Torr, the surface becomes coarse since SiHdesorbs.

In this embodiment, the aforementioned steps are sequentially performed.Although the aforementioned steps can be separately performed,preferably they are sequentially performed in order to form aninsulating film having a better film quality.

Furthermore, in this embodiment, it is preferable that the heattreatment in the H₂ atmosphere at step S10 be performed at a temperatureequal to or more than the temperature at which the heat treatment in theH₂ atmosphere at step S12 is performed.

As described above, in the fabricating method of this embodiment, it ispossible to set the coordination number of a nitrogen atom in theoxynitride layer to be three, thereby forming a flat interface at anatomic level to considerably decrease a small amount of energy levels inthe band gap. Accordingly, when an oxynitride layer formed in accordancewith the fabricating method of this embodiment is used as, for example,a gate oxynitride film of an MOS transistor, it is possible to form adevice performing constant and stable operations with the fluctuationsin threshold voltage being curbed and the degradation of mobility beingprevented.

The heat treatment in the H₂ atmosphere like step S10 of the sixthembodiment, which is performed before the nitiriding processing, canalso be performed before the nitriding processing in the first to fourthembodiments (FIG. 16). In such a case, unlike the sixth embodiment, theannealing after the nitriding is performed in a nitrogen gas atmosphereinstead of an H₂ atmosphere.

As described above, according to the embodiments of the presentinvention, it is possible to obtain an insulating film wherein theenergy levels in the band gap are reduced.

As in the case of the first embodiment, in a semiconductor devicefabricated in accordance with any of the second to sixth embodiment, theatoms that are the closest to a nitrogen atom in a three-coordinate bondstate are silicon atoms, and the atoms that are next closest are atomsother than silicon atoms, e.g., oxygen-atoms.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcepts as defined by the appended claims and their equivalents.

1. A method for fabricating a semiconductor device comprising: arrangingnitrogen atoms on a surface of a silicon substrate; performing a heattreatment in a hydrogen atmosphere with a hydrogen partial pressure of50 Torr or more so that the nitrogen atoms and silicon atoms existing onthe surface of the silicon substrate are brought into a three-coordinatebond state; and forming a silicon oxide film on the silicon substratewith the three-coordinate bond state of nitrogen atoms and the siliconatoms being maintained.
 2. The method for fabricating a semiconductordevice according to claim 1, wherein the heat treatment in the hydrogenatmosphere is performed at a temperature of 500° C. or more and 950° C.or less.
 3. The method for fabricating a semiconductor device accordingto claim 2, wherein atoms that are the closest to a nitrogen atom in thethree-coordinate bond state are silicon atoms, and atoms that are nextclosest are atoms other than silicon atoms.
 4. The method forfabricating a semiconductor device according to claim 3, wherein theatoms that are next closest are oxygen atoms.
 5. The method forfabricating a semiconductor device according to claim 1, wherein atomsthat are the closest to a nitrogen atom in the three-coordinate bondstate are silicon atoms, and atoms that are next closest are atoms otherthan silicon atoms.
 6. The method for fabricating a semiconductor deviceaccording to claim 5, wherein the atoms that are next closest are oxygenatoms.
 7. A method for fabricating a semiconductor device comprising:performing a heat treatment on a silicon substrate in a first hydrogenatmosphere; arranging nitrogen atoms on a surface of the siliconsubstrate; performing a heat treatment in a second hydrogen atmospherewith a hydrogen partial pressure of 50 Torr or more so that the nitrogenatoms and silicon atoms existing on the surface of the silicon substrateare brought into a three-coordinate bond state; and forming a siliconoxide film on the silicon substrate with the three-coordinate bond stateof the nitrogen atoms and the silicon atoms being maintained.
 8. Themethod for fabricating a semiconductor device according to claim 7,wherein the heat treatment in the first hydrogen atmosphere is performedat a temperature of 500° C. or more.
 9. The method for fabricating asemiconductor device according to claim 8, wherein the heat treatment inthe first hydrogen atmosphere is performed at a temperature of 500° C.or more, and the heat treatment in the second hydrogen atmosphere isperformed at a temperature of 500° C. or more and 950° C. or less, thetemperature of the heat treatment in the second hydrogen atmospherebeing higher than that in the first hydrogen atmosphere.
 10. The methodfor fabricating a semiconductor device according to claim 8, whereinatoms that are the closest to a nitrogen atom in the three-coordinatebond state are silicon atoms, and atoms that are next closest are atomsother than silicon atoms.
 11. The method for fabricating a semiconductordevice according to claim 10, wherein the atoms that are next closestare oxygen atoms.
 12. The method for fabricating a semiconductor deviceaccording to claim 7, wherein the heat treatment in the first hydrogenatmosphere is performed at a temperature of 500° C. or more, and theheat treatment in the second hydrogen atmosphere is performed at atemperature of 500° C. or more and 950° C. or less, the temperature ofthe heat treatment in the second hydrogen atmosphere being higher thanthat in the first hydrogen atmosphere.
 13. The method for fabricating asemiconductor device according to claim 7, wherein atoms that are theclosest to a nitrogen atom in the three-coordinate bond state aresilicon atoms, and atoms that are next closest are atoms other thansilicon atoms.
 14. The method for fabricating a semiconductor deviceaccording to claim 13, wherein the atoms that are next closest areoxygen atoms.